As current and future wireless communication networks transition away from circuit-switched communications and toward packet-switched communications, resource allocation becomes more flexible and more challenging. For example, in packet-switched environments, resource allocations tend to be made on a demand basis. Further, resource allocations to individual users tend to be of short duration, reflecting the bursty, intermittent nature of many packet-switched communication applications, such as web browsing, etc.
As one example, packet-based communication applications and services complicate the allocation of common uplink resources in cellular and other wireless communication networks, such as where a potentially large group of users in a given cell share a common communication uplink. One sharing approach depends on resource reservation techniques, wherein individual users access the common uplink on a reserved basis. Scheduling access to the communication link in this manner has the advantage of controlling the group of users in a manner that avoids conflicting transmissions (i.e., simultaneous transmission on the same physical resource by more than one user) but the scheduling request/grant signaling adds potentially significant overhead to the communication link, and adds complexity to the managing base station.
As an alternative, contention-based access offers potentially greater communication link efficiency by eliminating the signaling overhead of scheduled access. With contention-based access, a grant for access to a contention-based channel is broadcast to a cell. Any uplink time aligned user can respond to that grant by opportunistically transmitting on the contention-based channel, even if the user has not been allocated a grant on the schedule-based channel. With each user vying for access to the contention-based channel transmitting autonomously, collisions between user transmissions occur from time to time. Commonly, the base station or other receiving entity can demodulate data from more than one user's transmission at the same time, so collisions do not necessarily mean loss of user data. However, interference or practical constraints on processing resources generally limits the number of simultaneous user transmissions that can be successfully received.
Without provisions to stagger or otherwise disperse Hybrid Automatic Resend Request (HARQ) retransmissions on the contention-based channel, users involved in a collision may attempt retransmissions at or about the same time, thereby risking yet another collision. Such potentially reduces or eliminates the efficiency gains that come with the elimination of scheduling overhead from the communication link.